Method for directed self-assembly (dsa) of block copolymers

ABSTRACT

In directed self-assembly (DSA) of a block copolymer (BCP), a patterned sublayer on a substrate serves as a guiding chemical prepattern on which BCPs form more uniform and/or denser patterns. A layer of a blend of a BCP and functional homopolymers, referred to as inks, is deposited on the patterned sublayer and annealed to change the initial chemical prepattern to a 1:1-like chemical pattern that is more favorable to DSA. After annealing, the inks selectively distribute into blocks by DSA, and part of the inks graft on the substrate underneath the blocks. The BCP blend layer is then rinsed away, leaving the grafted inks A second layer of BCP is then deposited and annealed as a second DSA step to form alternating lines of the BCP components. One of the BCP components is removed, leaving lines of the other BCP component as a mask for patterning the substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the directed self-assembly (DSA) ofblock copolymers (BCPs), and more particularly to the DSA of BCPs tomake an etch mask for pattern transfer into a substrate.

2. Description of the Related Art

Directed self-assembly (DSA) of block copolymers (BCPs) has beenproposed for making imprint templates. Imprint templates haveapplication in making patterned-media magnetic recording disks and insemiconductor manufacturing, for example, for patterning parallelgenerally straight lines in MPU, DRAM and NAND flash devices. DSA ofBCPs by use of a patterned sublayer for the BCP film is well-known.After the BCP components self-assemble on the patterned sublayer, one ofthe components is selectively removed, leaving the other component withthe desired pattern, which can be used as an etch mask to transfer thepattern into an underlying substrate. The etched substrate can be usedas an imprint template.

With the prior art method of DSA of BCP thin films, the quality of theself-assembled pattern depends upon a number of factors, including filmthickness, the acceptable defect density, the density multiplicationfactor, the chemistry of the patterned sublayer and individual stripewidth of the patterned sublayer. The design of a process to have a largeprocess window for one parameter usually implies that the process windowfor the other parameters gets reduced or compromised. For example,thicker BCP films are desirable for pattern transfer into the template;however, a thicker film results in higher defect densities or in a lowertolerance for a high density multiplication factor. Conversely, if ahigher density multiplication factor is desired, or if more tolerance isneeded for the width of the stripes, the film thickness needs to bereduced considerably to avoid a large defect density.

What is needed is an improved method for DSA of BCPs that allows for alarge process window.

SUMMARY OF THE INVENTION

Embodiments of the invention relate to a method to improve the processof DSA of BCPs, especially on chemical prepatterns. For a DSA process, aconventional lithography process is used to generate a patternedsublayer as a guiding chemical prepattern on which BCPs form moreuniform and/or denser patterns. An additional DSA step uses a blend ofBCP and functional homopolymers to change the initial chemicalprepattern to a 1:1-like chemical pattern that is more favorable to DSA.The initial chemical prepattern can be generated by any conventionallithography process. Then a BCP blend with a small portion of functionalhomopolymers is deposited and annealed on the initial chemicalprepattern. A functional homopolymer, referred to as an ink, istypically the same as one of the BCP blocks. After annealing, the BCPblend will be guided by the initial chemical prepattern. The inks willselectively distribute into blocks, and part of the inks will graft onthe substrate underneath the blocks. The BCP blend layer is then rinsedaway, leaving the grafted inks When the grafted inks are selected to belonger than the brushes used in the initial chemical prepattern, theywill form 1:1 chemical patterns in some regions that have the samegeometry with BCP bulk morphology. Depending on the property of theinitial chemical prepattern, the modified chemical pattern can be afully 1:1, or a partially 1:1 chemical pattern. Either type of themodified chemical pattern will be acceptable for a second DSA of a BCPlayer with a thickness of typically greater than L₀. The resulting BCPlayer will serve as mask for patterning other layers.

The method can significantly widen the process window of the DSA incomparison to prior art methods. The wider processing window can befirst obtained by sacrificing one of the DSA parameters such as filmthickness, e.g., to use a thin BCP blend layer with thicknesssubstantially less than L₀. The thin BCP film can form defect-freepatterns that cannot be formed using a BCP film with a regular thicknessequal to or greater than L₀. The nearly perfect pattern will be“printed” on the initial chemical prepattern by the inks Then a secondDSA on the modified prepattern will be carried out without scarificationon any parameter. Therefore, the method can reduce the requirements forthe conventional lithography processes in terms of throughput, patternpitch, pattern critical dimension, and pattern roughness. The method canalso ease the strict restriction of the surface chemistry of theprepattern.

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the following detaileddescription taken together with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1A-1D are view illustrating the prior art method for making animprint template using directed self-assembly (DSA) of block copolymers(BCPs).

FIGS. 2A-2G are illustrations of an embodiment of the invention, whereinFIGS. 2A, 2B, 2D, 2F and 2G are perspective views of the structure atvarious stages of the process, FIG. 2C is an expanded sectional view ofa portion of FIG. 2B, and FIG. 2E is an expanded sectional view of aportion of FIG. 2D.

FIG. 3A is a perspective view showing a structure where L₀=27 nm andL_(S)=2L₀ with a thickness of the additional BCP layer being about 30nm.

FIG. 3B is a scanning electron microscopy (SEM) image of a top view ofthe structure of FIG. 3A but made according to the prior art method ofDSA.

FIGS. 3C-3H are SEM images of a top view of structures made according toan embodiment of the method of this invention for values of W of 0.57L₀,0.69L₀, 0.831L₀, 0.93L₀, 1.01L₀ and 1.09L₀, respectively.

FIG. 4A is a perspective view showing a structure where L₀=27 nm,L_(S)=5L₀, W=2.5L ₀ and with a thickness of the additional BCP layerbeing about 30 nm.

FIG. 4B is a SEM image of a top view of the patterned and etched e-beamresist with cross-linked polystyrene XPS stripes underneath, showing thegeometry of the chemical patterns with L_(S)=135 nm and W≅67.5 nm.

FIG. 4C is a SEM image of a top view of the additional BCP layer withalternating parallel PS and PMMA lines.

FIG. 4D is a SEM image of chromium (Cr) lines formed by removal of thePMMA lines and dry lift-off using the remaining PS lines as a mask.

FIG. 5A is a SEM image of a top view of the patterned and etched e-beamresist with (XPS) stripes underneath, showing the geometry of thechemical patterns with L_(S)=272.4 nm, W≅19 nm.

FIG. 5B is a SEM image of a top view of the additional BCP layer with athickness of about 30 nm on chemical patterns with L_(S)=266.1 nm.

FIG. 5C is a SEM image of a top view of the additional BCP layer with athickness of about 30 nm on chemical patterns with L_(S)=268.2 nm.

FIG. 5D is a SEM image of a top view of the additional BCP layer with athickness of about 30 nm on chemical patterns with L_(S)=270.0 nm.

FIG. 5E is a SEM image of a top view of the additional BCP layer with athickness of about 30 nm on chemical patterns with L_(S)=272.4 nm.

FIG. 5F is a SEM image of a top view of the additional BCP layer with athickness of about 30 nm on chemical patterns with L_(S)=274.2 nm.

FIG. 6A is a perspective view showing a structure where L₀=27 nm,L_(S)=nL₀, W=0.6L₀ and with a thickness of the additional BCP layerbeing about 2.08 L₀.

FIG. 6B is a SEM image of a top view of the additional BCP layer with athickness of about 56.25 nm on chemical patterns with L_(S)=162 nm.

FIG. 6C is a SEM image of a top view of the additional BCP layer with athickness of about 56.25 nm on chemical patterns with L_(S)=189 nm.

DETAILED DESCRIPTION OF THE INVENTION

Self-assembling block copolymers (BCPs) have been proposed for creatingperiodic nanometer (nm) scale features. Self-assembling BCPs typicallycontain two or more different polymeric block components, for examplecomponents A and B, that are immiscible with one another. Under suitableconditions, the two or more immiscible polymeric block componentsseparate into two or more different phases or microdomains on ananometer scale and thereby form ordered patterns of isolated nano-sizedstructural units. There are many types of BCPs that can be used forforming the self-assembled periodic patterns. If one of the components Aor B is selectively removable without having to remove the other, thenan orderly arranged structural units of the un-removed component can beformed.

Specific examples of suitable BCPs that can be used for forming theself-assembled periodic patterns include, but are not limited to:poly(styrene-block-methyl methacrylate) (PS-b-PMMA), poly(ethyleneoxide-block-isoprene) (PEO-b-PI), poly(ethylene oxide-block-butadiene)(PEO-b-PBD), poly(ethylene oxide-block-styrene) (PEO-b-PS),poly(ethylene oxide-block-methylmethacrylate) (PEO-b-PMMA),poly(ethyleneoxide-block-ethylethylene) (PEO-b-PEE),poly(styrene-block-vinylpyridine) (PS-b-PVP),poly(styrene-block-isoprene) (PS-b-PI), poly(styrene-block-butadiene)(PS-b-PBD), poly(styrene-block-ferrocenyldimethylsilane) (PS-b-PFS),poly(butadiene-block-vinylpyridine) (PBD-b-PVP),poly(isoprene-block-methyl methacrylate) (PI-b-PMMA),poly(styrene-block-lactic acid) (PS-b-PLA) andpoly(styrene-block-dymethylsiloxane) (PS-b-PDMS).

The specific self-assembled periodic patterns formed by the BCP aredetermined by the molecular volume ratio between the first and secondpolymeric block components A and B. When the ratio of the molecularvolume of the second polymeric block component B over the molecularvolume of the first polymeric block component A is less than about 80:20but greater than about 60:40, the BCP will form an ordered array ofcylinders composed of the first polymeric block component A in a matrixcomposed of the second polymeric block component B. When the ratio ofthe molecular volume of the first polymeric block component A over themolecular volume of the second polymeric block component B is less thanabout 60:40 but is greater than about 40:60, the BCP will formalternating lamellae composed of the first and second polymeric blockcomponents A and B. When the ratio of B over A is greater than about80:20 he BCP will form an ordered array of spheres in a matrix of thesecond component. For lamellar or cylinder forming BCPs, the orientationof the lamellae or the cylinders with respect to the substrate dependson the interfacial energies (wetting properties) of the block copolymercomponents at both the substrate interface and at the top interface.When one of the block components preferentially wets the substrate (orthe top free interface) the block copolymers form layers parallel to thesubstrate. When the wetting properties at the interface are neutral toeither block, then both block components can be in contact with theinterface, facilitating the formation of block copolymer domains withperpendicular orientation. In practice, the wetting properties of thesubstrate are engineered by coating the substrate with “surfacemodification layers” that tune the wetting properties at the interface.Surface modification layers are usually made of polymer brushes or matstypically (but not necessarily) composed of a mixture of the constituentblock materials of the BCP to be used.

The periodicity or natural pitch (L₀) of the repeating structural unitsin the periodic pattern BCP components is determined by intrinsicpolymeric properties such as the degree of polymerization N and theFlory-Huggins interaction parameter χ. L₀ scales with the degree ofpolymerization N, which in turn correlates with the molecular weight M.Therefore, by adjusting the total molecular weight of the BCP, thenatural pitch (L₀) of the repeating structural units can be selected.

To form the self-assembled periodic patterns, the BCP is first dissolvedin a suitable solvent system to form a BCP solution, which is thenapplied onto a surface to form a thin BCP layer, followed by annealingof the thin BCP layer, which causes phase separation between thedifferent polymeric block components contained in the BCP. The solventsystem used for dissolving the BCP and forming the BCP solution maycomprise any suitable non-polar solvent, including, but not limited to:toluene, propylene glycol monomethyl ether acetate (PGMEA), propyleneglycol monomethyl ether (PGME), and acetone. The BCP solution can beapplied to the substrate surface by any suitable techniques, including,but not limited to: spin casting, coating, spraying, ink coating, dipcoating, etc. Preferably, the BCP solution is spin cast onto thesubstrate surface to form a thin BCP layer. After application of thethin BCP layer onto the substrate surface, the entire substrate isannealed to effectuate microphase segregation of the different blockcomponents contained by the BCP, thereby forming the periodic patternswith repeating structural units.

The BCP films in the above-described techniques self-assemble withoutany direction or guidance. This undirected self-assembly results inpatterns with defects so it is not practical for applications thatrequire long-range ordering, such as imprint templates for bit-patternedmedia or integrated circuits. However, directed self-assembly (DSA) ofblock copolymers (BCPs) has been proposed for making imprint templatesfor bit-patterned media or integrated circuits. DSA of BCPs by use of apatterned sublayer that acts as a chemical contrast pattern for the BCPfilm is well-known, as described for example in U.S. Pat. No. 7,976,715;U.S. Pat. No. 8,059,350; and U.S. Pat. No. 8,119,017. Pendingapplication Ser. No. 13/627,492, filed Sep. 26, 2012 and assigned to thesame assignee as this application, describes the use DSA of BCPs to maketwo submaster imprint templates, one with a pattern of generally radiallines, and the other with generally concentric rings, to make a masterimprint template, which is then used to imprint patterned-media magneticrecording disks. Imprint templates made with DSA of BCPs have also beenproposed for use in semiconductor manufacturing, for example, forpatterning parallel generally straight lines in MPU, DRAM and NAND flashdevices.

The prior art method for making an imprint template using DSA of BCPswill be described in general terms with FIGS. 1A-1D for an example wherethe template 50 will become an imprint template with protrusions 51 in apattern of parallel bars. FIG. 1A is a side sectional view showing thetemplate 50 with a patterned sublayer of generally parallel stripes 105and intermediate substrate regions 106. Alternating A componentpolystyrene (PS) parallel lines 112 and B component (PMMA) parallellines 115 are formed over the stripes 105 and substrate regions 106. Theregions 106 can be exposed portions of the template 50 not covered bysublayer 105 or regions covered by a different sublayer. The sublayerhas been patterned to direct the self-assembly of the BCP A and Bcomponents with a natural pitch of L₀. In this example the stripes 105have a width W of 1.5L₀ and a stripe pitch L_(S) of 2L₀. In FIG. 1B, theportions of parallel lines 115, the B component (PMMA), are thenselectively removed by a wet etch or a dry etch process. This leavesgenerally parallel lines 112 of the A component (PS) on the template 50.Then, a dry etch process is used to etch the template 50 to formrecesses 52 using the parallel lines 112 as the etch mask. The materialof parallel lines 112 and the remaining underlying sublayer 105 is thenremoved, leaving recesses 52 in template 50. This leaves the structureas shown in FIG. 1C, with a pattern of protrusions formed as parallelbars 51 and recesses formed as parallel bars 52. FIG. 1D is a sidesectional view of the resulting imprint template.

With this prior art method of DSA of BCPs, the quality of theself-assembled pattern depends upon a number of factors, including theBCP film thickness, the acceptable defect density, the densitymultiplication factor “n” where L_(S)=nL₀, the chemistry of theprepatterns, and the width W of the individual stripes relative to L₀.The design of a process to have a large process window for one parameterusually implies that the process window for the other parameters getsreduced or compromised. For example, thicker BCP films (about 1-2 timesthe L₀) are desirable for pattern transfer into the template. However, athicker BCP film results in higher defect densities or in a lowertolerance for a high density multiplication factor. Conversely, if ahigher density multiplication factor is desired, or if more tolerance isneeded for the width of the stripes, the BCP film thickness needs to bereduced considerably to avoid a large defect density. In general, it isknown that when the chemical contrast patterns of the patterned sublayerare written at the same density as the resulting block copolymer pattern(i.e., L_(S)=L₀), the DSA results in the lowest defect densities withthe largest tolerance for stripe pattern variation or for a wide rangeof film thicknesses. However, it can be difficult to form a chemicalcontrast pattern as the patterned sublayer with such a small stripepitch. Thus it is desirable to have chemical contrast patterns with ahigh density multiplication factor (n greater than or equal to 2) thatcan still allow the use of thicker BCP films and result in low defectdensities.

Embodiments of the method of this invention use intermediate stepsbetween formation of the patterned sublayer and deposition of the BCPand thus replace the prior art method illustrated and described abovewith respect to FIG. 1A. The method is illustrated in FIGS. 2A-2F for anexample where the BCP is poly(styrene-block-methyl methacrylate)(PS-b-PMMA) with L₀=27 nm.

FIG. 2A is a perspective view of a substrate 200 with a patternedsublayer 205 that acts as a chemical contrast pattern. The substrate 200may be formed of any suitable material, such as, but not limited to,single-crystal Si, amorphous Si, silica, fused quartz, silicon nitride,carbon, tantalum, molybdenum, chromium, alumina and sapphire.

In case of the DSA of perpendicularly oriented lamellae, a patternedsublayer 205 is typically a periodic pattern of generally parallelstripes 206 with a stripe pitch L_(S)=nL₀ and alternate stripes 207. Oneof the stripes (206) is typically preferentially wetted by one of theblocks, which is called guiding stripes. The chemistry of the alternatestripes (207) is tuned accordingly, depending on the chemistry of theguiding stripes 206. When stripes 206 and 207 have approximately thesame height, they are typically referred to as chemical patterns. In theprior art, there are various types of chemical patterns specifically forPS-b-PMMA, which means different pairing of 206 and 207 stripes. Forinstance, the 206 and 207 stripes can be exposed silicon substrate and aPS-rich functionalized random copolymer PS-r-PMMA brush, respectively;exposed silicon substrate and a low molecular weight functionalized PSbrush, respectively; exposed silicon substrate and a PS-rich crosslinkedrandom copolymer PS-r-PMMA mat, respectively; crosslinked PMMA mat and aPS-rich functionalized random copolymer PS-r-PMMA brush, respectively;crosslinked PS mat and a PMMA-rich functionalized random copolymerPS-r-PMMA brush, respectively; e-beam resist HSQ (hydrogensilsesquioxane) and a PS-rich functionalized random copolymer PS-r-PMMAbrush, respectively; a functionalized PS brush and a PMMA-richcrosslinked random copolymer PS-r-PMMA mat, respectively; or afunctionalized PMMA brush and a PS-rich crosslinked random copolymerPS-r-PMMA mat, respectively.

A mat layer is a crosslinked polymer layer. The crosslinkable polymermay be spin-coated on the substrate to a thickness of 4-15 nm. Theas-spun film is then annealed or treated by UV light for thecross-linking units to carry out the cross-linking After cross-linking,the cross-linked polymer layer is typically referred as a mat layer. Thefilm thickness is similar to that of the as-spun layer. A brush layer isa monolayer of a functional polymer grafted on the substrate. Thefunctional polymer may be applied on the substrate to a thicknessgreater than 5 nm. The as-spun film is annealed for the functionalgroups to graft to the substrate surface. After annealing, any ungraftedbrush material is rinsed away in a suitable solvent (e.g., toluene,PGMA, or NMP). The thickness of the brush layer is typically 1-15 nm,which is determined by the properties of the functional polymer such aschemistry, molecular weight, location of the functional group, etc. Oneof the main differences between these two is that a mat layer is denserthan a brush layer and can prevent a further brush grafting on theunderlying substrate surface. In embodiments of this invention, sincethe additive brush grafting on the original chemical patterns isrequired, at least part of the chemical patterns is a brush layer orbare substrate surface.

For DSA, additional steps are required to create a chemical contrastpattern. These steps may include e-beam lithography, photolithography ornanoimprint lithography and potentially a combination of polymer matsand brushes. Referring again to FIG. 2A, in the present example, a layerof cross-linked polystyrene XPS (205) with a thickness of 5-8 nm isfirst formed on a silicon substrate, followed by deposition of an e-beamresist layer on top. E-beam lithography is utilized to generate gratingpatterns with a stripe pitch L_(S)=nL₀. The resist pattern is thenexposed to oxygen plasma etching so that the exposed portions of the XPSlayer are etched away. Meanwhile, the width of the resulted XPS stripes(206) is also tuned by lateral etching. The remaining resist pattern isrinsed away in a suitable solvent (e.g., toluene, PGMA, or NMP). Afunctionalized random copolymer “PS-r-PMMA” with a hydroxyl (OH) group(e.g., PS-r-PMMA-OH) consisting of ˜50% styrene is then spin-coated onthe substrate and annealed. Since XPS stripes are dense, PS-r-PMMA-OHcan only graft in the intermediate regions between XPS stripes (206).After rinsing away the ungrafted brush material in toluene or NMP, theremaining brush forms stripes 207 of grafted PS-r-PMMA-OH.

Next, in FIG. 2B and the expanded sectional view of FIG. 2C a solutionof a BCP blended with homopolymers with functional groups is deposited,for example by spin-coating, as a thin film 210 onto the chemicalpattern 205. Preferably the homopolymers with functional groups are thesame as the polymers in the BCP and the functional groups are functionalend groups. The preferred BCPs are PS-b-PMMA and polystyrene block poly2-vinylpyridine (PS-b-P2VP). The preferred functional groups for thehomopolymers are hydroxyl (OH) and amine (NH₂). This layer is depositedto a thickness in the range of 10 to 20 nm, substantially less than L₀.The functionalized homopolymers are sometimes called “inks” because theyare added to the blend.

In the example of FIGS. 2A-2G, the BCP is PS-b-PMMA and the homopolymersare OH-terminated PS (item 220) and OH-terminated PMMA (item 225). Theblend may be made up of 70-99% PS-b-PMMA and 30-1% inks The ratiobetween PS-OH and PMMA-OH is typically chosen as the same as the ratioof PS block and PMMA block in the BCP. This type of blend of a BCP withthe functionalized polymer inks 220, 225 has been used for moleculartransfer printing. In that process the original self-assembled patternthat results after the blend is annealed is copied or transferred to asecond top substrate. Molecular transfer printing is described inUS20090260750 A1 and S. Ji, et al., “Molecular Transfer Printing UsingBlock Copolymers”, ACS Nano 2010, 4, (2) 599-609.

Next in FIG. 2D and the expanded sectional view of FIG. 2E the film 210is annealed, for example by heating to 250° C. for at least 10 minutes.This results in a micro-phase separation (e.g., self-assembly intonanoscale domains) and allows the inks to be sequestered or phaseseparated into respective blocks of PS 240 and PMMA 245 within the BCP.Meanwhile, a fraction of the ink molecules will also react with thesubstrate regions 207, in this example the grafted PS-r-PMM-OH, thusbecoming immobile and creating a pattern with the same geometry andfeature size as the BCP pattern of PS 240 and PMMA 245. Since thethickness of BCP film 210 is substantially less than L₀, the DSA processwindow is much wider. For example, defect-free patterns can be achievedwith a density multiplication factor greater than 4, with lamellae pitchstretching or compressing for more than 5% for two times densitymultiplication, or with the width of XPS stripes greater than 0.7L₀ fortwo times density multiplication. In comparison, if the film thicknessis close to or greater than L₀, defective patterns are expected at thesame experimental conditions. However, the BCP film 210 is too thin tobe useful for subsequent pattern transfer into substrate 200.

After the film 210 has been annealed, it is rinsed in a suitablesolvent, for example in a solution of toluene or NMP, to remove the BCPand any functionalized polymers (inks) that are not bound to thesubstrate regions 207. This leaves the structure depicted in FIG. 2Fwith a patterned sublayer of stripes 206 of XPS mat material andintermediate regions 250 that contain the self-assembled pattern ofbound PS-OH and PMMA-OH shown in the sectional view of FIG. 2E. Afterrinsing, region 250 will change to 1:1 chemical pattern. If the width ofXPS stripes W≅0.5L₀, the original 1:n chemical pattern is modified to1:1. In cases of W≅1.5L₀, 2.5L₀, or other width greater than 0.5L₀, theresulted chemical pattern will be a 1:1-like chemical pattern. Overall,the 1:1 chemical pattern in regions 250 will greatly increase the DSAprocess window in terms of film thickness, density multiplicationfactor, commensurability tolerance, etc.

Since 210 is too thin to be useful for pattern transfer, thus in FIG. 2Gan additional layer 270 of BCP with a thickness greater than that oflayer 210 is deposited, for example by spin-coating, over the patternedsublayer of stripes 206 and regions 250, and the additional layer of BCPis annealed by heating to about 250° C. for at least 2 minutes. Thisadditional BCP layer 270 has a thickness preferably in the range of 25to 200 nm. The underlying pattern of stripes 206 and regions 250 directsthe BCP components to self-assemble into PS lines 260 and PMMA lines265. The PS lines 260 and PMMA lines 265 self-assemble as lamellaeperpendicular to the substrate.

If the BCP used in the solution deposited on the patterned sublayer inthe step of FIGS. 2B and 2C was blended with the homopolymers of theBCP, e.g., if the BCP contains copolymers A and B and the homopolymerswith functional groups are A and B, then this additional layer 270 ofBCP is the same BCP. Thus in the example of FIG. 2G, the additionallayer 270 of BCP is PS-b-PMMA. However, if the BCP used in the solutiondeposited on the patterned sublayer in the step of FIGS. 2B and 2C wasblended with different homopolymers than are in the BCP, e.g., if theBCP contains copolymers A and B and the honmopolymers with functionalgroups are polymers C and D, then this additional layer 270 is a BCPwith polymers C and D.

After the structure shown in FIG. 2G is formed according to embodimentsof the method of this invention, one of the BCP components can beremoved, leaving the other component as an etch mask for transferringthe pattern into substrate 200. These subsequent steps are known in theprior art, for example as shown and described above in FIGS. 1B-1D.

To determine the effect of ink length on the embodiments of the methodof this invention, a set of experiments was performed with inks ofvarying polymer lengths. The PS-r-PMMA-OH, PS-OH, and PMMA-OH used inthe experiments are all end-functionalized, thus the brush length isproportional to the molecular weight. The comparison between the lengthof PS-r-PMMA-OH and lengths of different inks can be tested by a watercontact angle experiment. First, PS-r-PMMA-OH brush is formed on siliconsubstrates. On each of these substrates, one ink (either PS-OH, orPMMA-OH) with certain molecular weight was additionally grafted. Thenthe water contact angles of all samples were measured and compared. Thecontact angle of the original PS-r-PMMA-OH brush layer was 78.5°. If thesubstrate is additionally grafted by PS-OH with molecular weight of 1.2,6, 10, or 17 kg/mol, the contact angle was 78.6°, 81.9°, 86.0°, or89.4°, respectively. The contact angle of PS is ˜89°, thus these resultsshow that PS-OH with molecular weight of 17 kg/mol is much longer, PS-OHwith molecular weight of 10 kg/mol is slightly longer, while PS-OH withmolecular weight of 1.2 or 6 kg/mol is shorter than PS-r-PMMA-OH. Thesame experiments were carried for PMMA-OH with molecular weight of 9.5or 30 kg/mol, and the contact angle changed to 73.4°, or 68.3°respectively. PMMA-OH with molecular weight of 30 kg/mol is much longer,while PMMA-OH with molecular weight of 9.5 kg/mol is slightly longerthan PS-r-PMMA-OH brush, since the contact angle of PMMA is ˜68°.

Another set of experiments was also performed to study the effect of theink length. A series of identical chemical patterns with densitymultiplication factors of 2 through 9 were generated. The guidingstripes 206 are XPS lines with width of ˜0.5L₀. The stripes 207 arePS-r-PMMA-OH containing 50% styrene, which is the same brush used forthe previous water contact angle experiments. Four samples of BCP orBCP-inks blend were used for layer 210 in FIG. 2B. Sample-A is purePS-b-PMMA without inks; Sample-B is 90% PS-b-PMMA, 5% PS-OH withmolecular weights of 6 kg/mol and 5% PMMA-OH with molecular weights of6.5 kg/mol; Sample-C is 90% PS-b-PMMA, 5% PS-OH with molecular weightsof 10 kg/mol and 5% PMMA-OH with molecular weights of 9.5 kg/mol; andSample-D is 90% PS-b-PMMA, 5% PS-OH with molecular weights of 17 kg/moland 5% PMMA-OH with molecular weights of 30 kg/mol. In all cases, thethickness of layer 210 is ˜b18 nm. After annealing, as shown in FIG. 2D,defect-free BCP pattern (layer 210) can be achieved on chemical patternswith density multiplication factor up to 6 or 7. Then the layer 210 isrinsed away in NMP, followed by deposition of 30 nm-thick pure BCP layer270. The BCP layer 270 is then annealed. As a control, when no ink isused in layer 210, the maximum density multiplication factor is onlyabout 3-4. When the inks are shorter than the PS-r-PMMA-OH, the 1:1chemical pattern cannot form in regions 250, and the maximum densitymultiplication factor is about 3-4, similar to the control experiment.When the inks are longer than the PS-r-PMMA-OH, the longer moleculesextend above the PS-r-PMMA-OH molecules to a height sufficient to allowfor binding to the BCP components when the additional layer of BCP isdeposited. The 1:1 chemical pattern can form in regions 250 and increasethe DSA process window, e.g., the maximum density multiplication factoris 6-7, which is significantly better than the control experiment. Thusin the preferred embodiments the PS-OH and PMMA-OH molecules should belonger than the PS-r-PMMA-OH molecules. The length of a polymer brushhighly depends on the chemistry, position of the functional group (atthe end of the chain, on the side chain, on the main chain), and numberof the functional groups. Typical molecular weight of a polymer brushused for DSA is ˜1-100 kg/mol. In cases of using chemical prepatternscontaining bare substrate surface, there is no requirement on the inklength.

DSA of BCP films with thickness greater than L₀ on chemical patternswith wider range of W was demonstrated according to embodiments of themethod of this invention. FIG. 3A is a perspective view of showing astructure where L₀=27 nm and L_(S)=2L₀ with a thickness of theadditional BCP layer being about 30 nm (equivalent to FIG. 2G). FIG. 3Bis a scanning electron microscopy (SEM) image of a top view of thisstructure but made according to the prior art method of DSA, and clearlyshows that the completed lines of PS and PMMA are not close to beingparallel over any extended length. In contrast, FIGS. 3C-3H are SEMimages of a top view of structures made according to embodiments of themethod of this invention for values of W of 0.57L₀, 0.69L₀, 0.83L₀,0.93L₀, 1.01L₀ and 1.09L₀, respectively. The inks used here were 5%PS-OH with molecular weights of 10 kg/mol and 5% PMMA-OH with molecularweights of 9.5 kg/mol. As shown, for FIGS. 3C-3F, all of the lines arenearly perfectly parallel without defects over an extended length.

Previous studies have shown that DSA works well when the width of theXPS guiding stripes W≅0.5L₀ or 1.5L₀, with 1 PS lamellae or 2 PSlamellae and 1 PMMA lamellae on top of the XPS guiding stripes,respectively. According to embodiments of the method of this invention,DSA works well also when W≅2.5L₀, with 3 PS lamellae and 2 PMMA lamellaeon top of the XPS guiding stripes. FIG. 4A is a perspective view showinga structure where L₀=27 nm, L_(S)=5L₀, W≅2.5L₀, n=5 and with a thicknessof the additional BCP layer being about 30 nm. FIG. 4B is a SEM image ofa top view of the oxygen plasma etched e-beam resist pattern with XPSstripes (206 in FIG. 2A) underneath. The resist pattern is then rinsedaway and the intermediate regions between stripes 206 are then graftedwith PS-r-PMMA-OH brush (stripes 207). FIG. 4C is a SEM image of a topview of the additional BCP layer (layer 270 in FIG. 2G) with alternatingparallel PS lines and PMMA lines. FIG. 4D is a SEM image of chromium(Cr) lines formed by removal of the PMMA lines, deposition of a Crlayer, and dry lift-off of the Cr lines on top of the PS lines. FIGS.4A-4D illustrate that embodiments of the method of the invention can beused to form generally parallel lines in a substrate for a relativelylarge stripe width (W=2.5L₀) and a relatively large densitymultiplication factor (n=5). Based on these experimental results it isbelieved that W can have a width up to one-half the densitymultiplication factor times L₀, i.e., W=(n/2)L₀.

DSA with a high density multiplication factor was demonstrated accordingto embodiments of the method of this invention. A series of chemicalpatterns with density multiplication factors of 10 were generated. Thechemical patterns have similar XPS guiding width W of 18-20 nm,identical PS-r-PMMA-OH brush as stripes 207, but various L_(s) values of266.1, 268.2, 270, 272.4, and 274.2 nm. FIG. 5A is a SEM image of a topview of the oxygen plasma etched e-beam resist pattern that was used tocreate the chemical pattern. FIGS. 5B-5F are top-down SEM images of theadditional BCP layer (layer 270 in FIG. 2F) guided by chemical patternswith L_(s) of 266.1, 268.2, 270, 272.4, and 274.2 nm, respectively.Nearly perfect BCP patterns are observed on chemical patterns with L_(s)of 266.1, 268.2, 270, and 272.4 nm. When L_(s) is 274.2 nm, a defectiveBCP pattern is observed.

Formation of a BCP layer with a thickness more than 2L₀ on chemicalpatterns with high density multiplication factor was demonstratedaccording to embodiments of the method of this invention. FIG. 6A is aperspective view showing a structure where L₀=27 nm, L_(S)=nL₀, W≅0.6L₀and with a thickness of the additional BCP layer being about 2.08L₀.FIGS. 6B and 6C are top-down SEM images of the additional BCP layer(layer 270 in FIG. 2F) with a thickness of 56.25 nm guided by chemicalpatterns with L_(S) of 162 and 189 nm, respectively.

Thus the use of the thin film of bound self-assembled PS-OH and PMMA-OHas the patterned sublayer for the subsequent DSA of the additional BCPlayer allows for a large design process window. A wide range of densitymultiplication factors and stripe widths can be used in the DSA to formnearly perfectly parallel lines of PS and PMMA with minimal defects.

While the present invention has been particularly shown and describedwith reference to the preferred embodiments, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.Accordingly, the disclosed invention is to be considered merely asillustrative and limited in scope only as specified in the appendedclaims.

What is claimed is:
 1. A method of directed self-assembly (DSA) of ablock copolymer (BCP) having a natural pitch L₀ comprising: providing asubstrate; forming on the substrate a patterned sublayer with generallyparallel stripes having a stripe pitch L_(S) of approximately nL₀, wheren is an integer greater than or equal to 1; depositing on the patternedsublayer a solution comprising a first BCP of polymers A and B withoutfunctional groups, a polymer C with a functional group and a polymer Dwith a functional group; heating said deposited solution to causepolymers C and D to self assemble and bind to the patterned sublayer;removing the first BCP and unbound polymers, leaving the self-assembledbound polymers C and D; depositing on the self-assembled bound polymersC and D a layer of a second BCP of polymers C and D without functionalgroups; and heating said layer of second BCP to cause DSA of the secondBCP into alternating lines of polymers C and D, said alternating lineshaving a line pitch of approximately L₀.
 2. The method of claim 1wherein polymer A is identical to polymer C and polymer B is identicalto polymer D, whereby the first BCP is identical to the second BCP. 3.The method of claim 1 wherein the polymers C and D with functionalgroups are copolymers of a BCP.
 4. The method of claim 1 wherein polymerA is polystyrene (PS), polymer B is poly(methyl methacrylate) (PMMA),polymer C is PS and polymer D is poly 2-vinylpyridine (P2VP).
 5. Themethod of claim 1 wherein polymer A is PS, polymer B is P2VP, polymer Cis PS and polymer D is PMMA.
 6. The method of claim 1 wherein thestripes have a width W greater than or equal to 0.5L₀ and less than orequal to (n/2)L₀.
 7. The method of claim 1 wherein n is greater than orequal to 2 and less than or equal to
 10. 8. The method of claim 1wherein depositing the layer of second BCP comprises depositing thelayer of second BCP to a thickness of at least 2L₀.
 9. The method ofclaim 1 wherein the functional groups are selected from OH and NH₂. 10.The method of claim 1 further comprising, after heating said layer ofsecond BCP to cause DSA of the BCP into alternating lines of polymers Cand D, patterning the substrate using the lines of one of polymers C andD as a mask.
 11. The method of claim 1 wherein said alternating lines offirst and second polymers form a pattern selected from parallelgenerally straight lines, generally radial lines, and generallyconcentric circular lines.
 12. A method of directed self-assembly (DSA)of a block copolymer (BCP) having a natural pitch L₀ comprising:providing a substrate; forming on the substrate a patterned sublayerwith generally parallel stripes having a stripe pitch L_(S) ofapproximately nL₀, where n is an integer greater than or equal to 1;depositing on the patterned sublayer a solution comprising a firstpolymer having a functional group, a second polymer having a functionalgroup, and a BCP of the first polymer without a functional group and thesecond polymer without a functional group; heating said depositedsolution to cause the first and second polymers with functional groupsto self assemble and bind to the patterned sublayer; removing the BCPand unbound polymers, leaving the self-assembled bound first and secondpolymers with functional groups; depositing on the self-assembled boundfirst and second polymers with functional groups a layer of BCPidentical to the BCP in said previously deposited solution; and heatingsaid layer of BCP to cause DSA of the BCP into alternating lines offirst and second polymers, said alternating lines having a line pitch ofapproximately L₀.
 13. The method of claim 12 wherein forming thepatterned sublayer comprises depositing a layer of polymer brushmaterial on the substrate, depositing resist on the polymer brushmaterial, patterning the resist with e-beam, etching the polymer brushmaterial and removing the resist, wherein the patterned sublayercomprises stripes of polymer brush material and exposed substratebetween said stripes.
 14. The method of claim 12 wherein forming thepatterned sublayer comprises depositing a mat of cross-linkedpolystyrene (PS) on the substrate, depositing an e-beam resist on themat, patterning the resist with e-beam, etching the mat and removing theresist leaving stripes of mat and exposed substrate between saidstripes, and depositing a random copolymer brush material comprising PSand poly(methyl methacrylate) (PMMA) having an OH end group(PS-r-PMMA-OH) to cause the PS-r-PMMA-OH to bind to the exposedsubstrate.
 15. The method of claim 14 wherein the first polymer having afunctional group is PS-OH and the second polymer having a functionalgroup is PMMA-OH, and each of said PS-OH and PMMA-OH polymers formbrushes on the substrate surface higher than the PS-r-PMMA-OH brushmaterial.
 16. The method of claim 12 wherein the stripes have a width Wgreater than or equal to 0.5L₀ and less than or equal to (n/2)L₀. 17.The method of claim 12 wherein n is greater than or equal to 2 and lessthan or equal to
 10. 18. The method of claim 12 wherein depositing thelayer of BCP identical to the BCP in said previously deposited solutioncomprises depositing to a thickness of at least 2L₀.
 19. The method ofclaim 12 wherein the functional groups are selected from OH and NH. 20.The method of claim 12 wherein the BCP is a copolymer of polystyrene(PS) and poly(methyl methacrylate) (PMMA).
 21. The method of claim 20wherein the first polymer having a functional group is PS-OH and thesecond polymer having a functional group is PMMA-OH.
 22. The method ofclaim 20 wherein the PS and PMMA self-assemble as lamellae perpendicularto the substrate.
 23. The method of claim 12 further comprising, afterheating said layer of BCP to cause DSA of the BCP into alternating linesof first and second polymers, patterning the substrate using the linesof one of the first and second polymers as a mask.
 24. The method ofclaim 12 wherein said alternating lines of first and second polymersform a pattern selected from parallel generally straight lines,generally radial lines, and generally concentric circular lines.